1. Field of the Invention
This invention relates to a developing method and a developing unit which renders an electrostatic latent image visible in electrophotographic devices or electrostatic recorders. More particularly, it is directed to a developing method which can provide a high quality image using a single component toner and a developing unit suitable for applying such a developing method.
2. Description of the Related Art
As a developing method using a single component toner (developing medium), a pressure developing method has been known by specifications such as those of U.S. Pat. Nos. 3,152,012; 3,754,963; and 3,731,146, and publications such as Japanese Patent Laid Open Nos. 13088/1972 and 13089/1972, Japanese Patent Publications Nos. 36070/1976 and 36414/1977. This pressure developing method is characterized by forming a thin layer of single component developing medium composed solely of a nonmagnetic toner on the surface of a toner carrier which is elastic, conductive and roughened; and bringing this toner layer into contact with the surface of an electrostatic latent image holding body which holds an electrostatic latent image in such a manner that their relative speeds become zero. Its advantage includes a simpler device and an easy color image production. However, from the results of the additional tests conducted by the present inventor and his group the following problems were addressed.
(1) The aforesaid pressure developing method is characterized principally as moving both surfaces of the toner layer and electrostatic image relative to each other at a circumferential speed of substantially zero. However, the tests indicated that images developed under the above condition lacked in sharpness and suffered from fog on non-image portions and nonuniform density. In contrast thereto, when a certain speed difference was given, the toner particles rolled and slipped at the position where the toner layer contacted the electrostatic latent image; and this encouraging electrification of toner particles and adjustment of image formation, produced the extremely sharp, consistent and high density developed images that were free from fog on non-image portions.
(2) In the pressure developing method, the electrically charged particles, or toner, on the toner carrier is caused to transfer to the electrostatic latent image, so that current (hereinafter referred to as "developing current") flows in the electric circuit between the toner carrier and the developing bias power supply. Thus, it is necessary to adjust a resistance on the toner carrier surface or a resistance between the toner carrier surface and the developing bias power supply to above a predetermined value. In the aforesaid prior art, however, there was no disclosure of a practicable concept relevant to this point.
(3) Moreover, the current, whose flow is caused mainly by the transfer of the toner particles, varies depending on such factors as the quantity of toner electrification, the quantity of toner adhesion to the electrostatic latent image formed on the surface of the electrostatic latent image holding body, the speed of movement of the toner carrier surface and the dimensions of the toner carrier. Therefore, the relationship between these factors and the above resistance may cause the variation in a potential on the surface of the toner carrier, i.e., an effective developing bias, and such a variation may in some cases impair the developed images with fog and insufficient density.
(4) Compared to a method utilizing magnetization to attract and carry magnetic toner on the surface of the toner carrier, this pressure developing method has difficulty in carrying the nonmagnetic developing medium (toner) on the surface of the toner carrier and then constantly supplying a predetermined quantity thereof to a latent image. Because for the nonmagnetic toner there is no remotely acting force such as a magnetic force that ensures to form and recover the thin layer of toner on the surface of the toner carrier when the toner layer has been consumed from the toner layer surface by the development of a predetermined latent image (the capability of quickly recovering the toner thin layer on the toner carrier and constantly supplying a predetermined quantity of toner thin layer to the latent image is hereinafter referred to as "toner transferability"). Defects in toner transferability impairs density in the latter half of a developing process of making a solid image. Thus, in order to improve the toner transferability, a sponge roller or a brush roller is disposed in a toner container. A method of rubbing the nonmagnetic toner on the toner carrier by the above roller to thereby supply it is disclosed, e.g., in Japanese Patent Laid Open Nos. 5274/1987, 7067/1987 and 95558/1987.
(5) In the aforesaid pressure developing method, the toner carried by the toner carrier is pressed on or put in contact with the electrostatic latent image for development, and this requires that a developing roller that is elastic and conductive be used as a toner carrier. If the electrostatic latent image holding body is made of a rigid body, it is essential that the toner carrier is formed of an elastic body in order to avoid damaging the electrostatic latent image holding body.
A known example of a toner carrier thus formed is a developing roller, in which the surface of a metal roller base material is provided with an elastic body layer such as a foam rubber or a polyurethane foam, and further with a flexible conductor layer and an outermost layer having graphite particles dispersed in a binding resin successively coated one upon the other (Japanese Patent Laid Open No. 13088/1972). More specifically, a toner carrier (developing roller) whose surface layer is coated with the above-mentioned mixture of graphite and a binding resin, using a horizontally coating machine, to a thickness of about 20 .mu.m on a polyethylene terephthalate thin plate that has been subjected to a chemical processing by aluminum.
(6) Moreover, since a thin layer of toner is formed on the toner carrier in the pressure developing method, means for pressing a toner layer forming member on the toner carrier is employed. As this toner layer thickness regulating means, the following two types are generally known.
(a) The middle part of a platelike toner layer thickness regulating member is pressed on the toner carrier.
(b) The end part of a platelike toner layer thickness regulating member is pressed on the toner carrier.
The method or means (a) in which the middle part of the platelike toner layer thickness regulating member is pressed is disclosed in, e.g., Japanese Patent Publication No. 16736/1988, Japanese Patent Laid Open Nos. 165866/1982 and 73649/1985 and 138967/1986, and in the specification of U.S. Pat. No. 4,521,098. In this method, the middle part of the platelike regulating member made of an elastic body is pressed not only to form a toner thin layer of uniform thickness but also to properly triboelectrify toner particles to thereby allow a satisfactory visible image to be produced.
On the other hand, the means (b) in which the end part of the platelike toner layer thickness regulating member is pressed is disclosed in such publications as Japanese Patent Publication No. 36070/1976 and 15068/1985, and Japanese Patent Laid Open Nos. 23638/1978 and 116559/1983, 95559/1987, 96981/1987 and 113178/1987. These known means for pressing the end part are classified into the following three types.
(i) Method of pressing a tip formed into a cylindrical surface (Japanese Patent Publication No. 36070/1976).
(ii) Method of pressing a tip that is sharp (Japanese Patent Laid Open No. 23638/1978 and others).
(iii) Method of pressing a tip formed into a plane surface (Japanese Patent Laid Open No. 95559/1987 and others). According to these methods, it is possible to form a desired toner thin layer with a relatively low pressing force, thereby allowing to overcome various problems associated with the method (a) of pressing the middle part. However, these methods (b) have the following problems. In the case where a sharp tip is pressed such as in method (ii), a strict pressure control is required to properly handle pressure concentration, which is caused by a very small area of contact between the toner carrier and the regulating member. A slightest inaccuracy in machining the tip resulted in inconsistency of the toner layer, and there was a tendency that the formed toner layer was excessively thin. In case of method (iii), the section of the end part of the platelike regulating member is pressed on the toner carrier, therefore there is no such problems as presented in method (ii) in the normal condition, but if slight variations in the state of mounting the regulating member cause the edge of the section of the end part to contact the toner carrier, the problems similar to those in method (ii) may be caused. On the other hand, in method (i), there is no sharp edge is found in the regulating member. Therefore, no problems such as entailed in methods (ii) and (iii) by small variations in its mounting conditions will be caused, and thus the manufacture and assembly of the device can be facilitated. If the end part is curved, the effect that is intermediate between the effect of the means for pressing the middle part and that of the means for pressing the sharp end part can be obtained, thus forming a thin layer of toner and charging toner particles at a comparatively lower pressure.
As to the problem (1), Japanese Patent Publication No. 12627/1985 and Japanese Patent Laid Open No. 23638/1978 and others disclose that a better quality image can be produced by moving the toner carrier faster than the electrostatic latent image. As to the problem (2), many proposals have been made on a preferable range of volume resistivity of the toner carrier surface. Japanese Patent Publication No. 22352/1985 has proposed use of a conductive toner carrier of below 10.sup.5 .OMEGA..multidot.cm; Japanese Patent Publication No. 3949/1987 below 10.sup.8 .OMEGA..multidot.cm; Japanese Utility Model Publication No. 35097/1987, above 10.sup.13 .OMEGA..multidot.cm; and Japanese Patent Publication No. 26386/1988, about 10.sup.8 .OMEGA..multidot.cm, respectively. However, such a differently set range of resistance is suggestive of possible variations of the optimal condition of development due to factors indicated in the problem (3), so that it will be difficult to produce a satisfactory developed image unless considerations are given to balancing these factors on an integrated basis.
As to the problem (4), the toner transferability could be improved to some extent; but in the case of inadequate triboelectrification between the toner carrier surface and the nonmagnetic toner particles, the nonmagnetic toner particles cannot adhere to the toner carrier surface, thereby leaving no chance of improving the transferability. Although the transferability is acceptable at an initial stage, it is often subjected to deterioration in the long run as the triboelectrification between the toner carrier surface and the nonmagnetic toner particles becomes inadequate due to a so-called "filming", or a phenomenon in that the nonmagnetic toner thin film is formed on the toner carrier surface.
By the way, generally known methods of controlling the density of an image to be produced in electrophotographic devices such as copying machines and laser printers involve control of the quantity of light for exposing an electrostatic latent image or of the developing bias for being applied to the toner carrier. These methods allow image density to be controlled to a certain extent; but if these methods applied to a developing method in which an image is developed by forming a thin layer of nonmagnetic toner on the toner carrier and supplying this toner layer to an electrostatic latent image, there is an upper limit in the obtained image density, and thus it is in no way possible to further increase it. It is because there is no further supply of toner to improve the density once all the thin layer of nonmagnetic toner has been consumed. An attempt to increase the thickness of the nonmagnetic toner layer to improve the density causes the nonmagnetic toner particles that adhere to the surface of the toner carrier to "fog" the non-image portions without going through a process of contacting the toner carrier, the toner layer thickness regulating member and the toner supplying member.
As to the problem (5), even if the elastic body layer satisfied the aforesaid condition, compression set occurred on the elastic body layer and gave, in some cases, adverse effects on the image when the elastic body layer was left under pressure for a long while. On the other hand, an elastic material that is less subject to compression set generally has a larger hardness; and if the toner carrier became eccentric, it was not easy to obtain a development nip width for covering the variation due to eccentricity, thereby inviting inconsistency in image density. Further, another difficult problem is that the smoothening of the surface of the toner carrier depends on the surface condition of the elastic body layer that forms its underlayer; i.e. the surface forming condition suitable for its material.
As to the durability of the toner carrier, there was no specific disclosure that gave a solution to the problem that a toner carrier with a conductive layer formed on the elastic body layer was subjected to damage, wear or flaking of the conductive layer during its use. Thus, not knowing the proper durability of a conductive layer, toner carriers that are too expensive to provide a required life were manufactured; the required life was not satisfied; or manufacturing control was so difficult that there was a noticeable inconsistency per lot.
A toner carrier that is made of an elastic material will provide a variety of practical advantages but bring the following disadvantages as well. When a toner layer forming member is pressed to form a thin layer of toner of a desired thickness, the pressed portion is hollowed to cause a so-called compression set. This defect tends to occur not only when one part of the toner carrier is continuously pressed for a long period of time but also at high or low temperatures. Once the compression set occurs, both the toner layer and the developing electric field at the development are subjected to being nonuniform, or it is made difficult to move the toner carrier and the latent image holding body at a constant speed. This gives a developed image nonuniform density and white and black stripes. Still worse is the fact that once the compression set is present, the toner carrier, even if used for the first time, may produce poor images. Thus, it is desired that a better environment should be ensured when the developing unit is warehoused or shipped.
As to the problem (6), in the developing units which press the middle part of the platelike regulating member (a), the toner particles are more likely to stay in a wedge-shaped space formed between the regulating member and the toner carrier. Since the incoming toner particles tend to press them out, it is required that a comparatively high pressure be employed to press the toner carrier to form a thin layer of toner of a desired thickness. This entailed the problems that the toner adhered to the toner carrier or the regulating member, and that a large force was required for driving the toner carrier.
It was found also that even the most practicable method (i) of pressing the end part formed into a cylindrical surface, among methods of pressing the end part of the platelike regulating member (b), suffered from the following problems. For example, Japanese Patent Publication No. 36070/1976 states that a regulating member which is made of polytetrafluoroethylene or polyformaldehyde (DELRIN.RTM.) and whose end part is formed into a cylindrical surface is suitable. However, from the additional tests conducted by the inventor and his group it was found that there were shortcomings such as inconsistency in the toner layer caused by inaccuracy in forming the regulating member, especially warpage and undulations along its length; inability of offsetting the mounting and forming inaccuracy of the regulating member because its material is nearly rigid; and difficulty in forming an accurate cylindrical surface. There was a tendency that the toner is gradually deposited on the surface of the regulating member by its use over a long period, thus inviting the toner layer inconsistency.
It is therefore an object of the present invention to provide a developing method which is capable of easily producing a high quality image that is sharp and free from fog on non-image portions.
A second object of the present invention is to provide a developing method which is capable of easily producing a uniform, high density image.
A third object of the present invention is to provide a developing method which is capable of easily producing a uniform, high density image by constantly forming and holding a predetermined toner layer on the surface of a toner carrier.
A fourth object of the present invention is to provide a developing unit which is capable of constantly producing a high-definition developed image free from nonuniform density or fog on non-image portions.
A fifth object of the present invention is to provide a developing unit which is capable of constantly producing a high-definition developed image free from nonuniform density or fog on non-image portions by forming and holding a consistent toner layer on the toner carrier.